The invention concerns printing devices. It concerns in particular a method, a control circuit, a computer program product and a printing device for an electrophotographic process with temperature-compensated discharge depth regulation.
An electrophotographic printing device is, for example, known from WO 00/41038. Information is thereby transmitted via a plurality of light sources (LED comb) or a luminosity-modulated laser beam onto a photoelectric layer (photoconductor), and therewith generates a charge image on the photoconductor. The latent charge image then passes through a developer station in which regions of different charge of the photoconductor are inked differently with toner. To stabilize such a developer process in spite of different operating conditions of the printing system, in particular temperature fluctuations of diverse components important to the electrophotographic process, it is important to bring to as uniform a value as possible the potential height of the locations of the photoconductor discharged via light. For this, the potential difference between the discharge depth of the exposed image locations and the potential level of the developer step is significant. While the potential level of the developer step can be regulated in a purely electrical manner, the influence factors for the potential difference are complex; in particular the luminous power of the light-generating device (character generator) with its influencing variables and the sensitivity of the photoconductor thereby play important roles. With the increasing process speed of electrophotographic printers, under otherwise identical boundary conditions the necessity increases to operate the light sources with increased energy, because the residence time is less due to the increased process speed and because the temporal separations of the sequential subprocesses between exposure and development of a latent charge image are reduced. The discharge via light does not occur abruptly in the region of the interaction, but rather approximately exponentially over time, conditional upon charge transport effects. A possibility to increase the available luminous power is to displace the working point for the emitted luminous power of the light source. Given LED combs as light sources, this means that the compensation for the uniformity of the light emission over the width of the comb must either be implemented in a shortened luminous duration (given the same light energy) or, however, implemented given an increased driver current of the light-emitting diodes. The possibility to use increased driver current is, however, only conditionally possible, since light-emitting diodes have a characteristic dependency of the luminous intensity on the temperature of the diode. Therefore a temperature compensation is necessary and the temperature compensation of the light yield requires a known additional upper margin. Furthermore, a current increase also affects the stability of the light-emitting diode over the lifespan, and thus the lifespan itself.
It is known from “Das Druckerbuch, Technik und Technologien der Drucksysteme”, Dr. Gerd Goldmann (Hsg.), Océ Printing system GmbH, 6th edition (May 2001), ISBN 3-00-001019-X, Chapter 2.2.4, page 5–22 to compensate the light strength of character generator light-emitting diodes via the luminous duration of the individual diodes. An individual luminous strength is therewith ensured over the width of the comb. The luminous duration times can be defined as a multiple of the periods of a set compensation frequency; a scaling of this frequency thereby leads to a scaling of the luminous duration, whereby the uniformity of the compensation can be (exactly) maintained. The variable (what is known as a) time base clock frequency exhibits two extreme values that, on the one hand, are defined upwards via the hardware-technical properties of the conductions on the character generator comb (conduction reflections) and, on the other hand are defined downwards by the necessity to be able to accommodate within a micro-row the correspondingly scaled, complete time scale from the compensation, meaning for example 255 periods of the time base clock (TBC). Since the time for the writing of a micro-row is dependent on speed, a speed-dependent lower boundary frequency thus also occurs.
A printing device with a photosensitive body is known from JP-A-03-289 681, in which the light quantity with which the photosensitive body is exposed is controlled. The control occurs dependent on a test exposure in which the actual surface potential of the photosensitive layer is determined, such that its changes are compensated based on temperature variations or changes of humidity.
A laser printing device with a light-sensitive body is known from EP-A2-210 077 in which the sensitivity of the light-sensitive body has a positive temperature characteristic.
A printing device with a photoconductor is known from JP-A-05-107 888 in which the surrounding temperature of the photoconductor is measured and is adjusted dependent on the measured temperature of the exposure strength.
It is known from DE-A1-35 343 38 that light-emitting diodes have a temperature response, and that this is compensated in an electrophotographic printer in which the diode trigger signals are varied depending on the temperature of the diodes.